Alumina-based sintered material containing alumina as a predominant component is excellent in withstand voltage characteristic, heat resistance, mechanical strength, etc. and is an inexpensive material. Therefore, the sintered material is employed for producing ceramic products, such as a spark plug ceramic insulator (may be referred to simply as “insulator” in the specification) and a multi-layer wiring board of IC packages. Such alumina-based sintered material has been produced by sintering a powder mixture containing a sintering aid; e.g., a 3-component sintering aid represented by SiO2—CaO—MgO.
When a spark plug insulator is produced from an alumina-based sintered material produced through sintering a raw material with the aforementioned 3-component sintering aid, after sintering, the sintering aid (mainly an Si component) remains at the grain boundary among alumina crystal grains as a low-melting-point glass phase. In this case, the low-melting-point glass phase is softened in a spark plug use environment, such as in a high temperature environment of about 700° C., impairing the withstand voltage characteristics of the insulator. The amount of low-melting-point glass phase in an alumina-based sintered material can be reduced by reducing the amount of the sintering aid added to the raw material. However, in this case, the insulator cannot be densified, or, even when the insulator has been apparently densified, a large number of pores remain in the grain boundary defined by alumina crystal grains, also impairing the withstand voltage characteristics of the insulator.
As described above, conventional alumina-based sintered materials include, at the grain boundaries, a low-melting-point glass phase or pores (residual pores). Thus, when a high voltage for generating spark discharge is applied to a spark plug having an insulator made therefrom in a high temperature environment (e.g., about 700° C.), the low-melting-point glass phase is softened, or the electric field is exclusively applied to the residual pores, possibly causing breakdown (penetration of spark) of the insulator.
For preventing such impairment in withstand voltage characteristic and/or breakdown, there have been proposed a spark plug insulator and a material thereof. For example, Japanese Patent Application Laid-Open (kokai) No. 2001-2464 discloses “a high-withstand-voltage alumina-based sintered material containing at least a rare earth element (hereinafter abbreviated as RE) component, characterized in that the alumina-based sintered material has a ratio of measured density to theoretical density of 95% or higher.”
Japanese Patent Application Laid-Open (kokai) No. 2001-335360 discloses an insulator for a spark plug, characterized in that the insulator comprises an Al component in an oxide-converted amount of 95 to 99.8 mass % with respect to the total amount of the constitutional components as 100 mass %, and a rare earth element and an Si component in such amounts that the ratio (RRE/RSi) of oxide-converted rare earth element content (RRE) to oxide-converted Si component content (RSi) is adjusted to 0.1 to 1.0, and that a cross-section having an area of 1 mm2 contains fewer than 10 alumina particles which have a longest size of 10 μm or longer and an aspect ratio of 3 or higher.”
International Publication WO 05/033041 discloses “an alumina ceramic composition containing alumina as a predominant component, characterized in that the composition is a complex sintered material comprising alumina serving as the predominant component and at least one element selected from among Al, Si, Mg, and rare earth elements and that the amount of said at least one element selected from among Al, Si, Mg, and rare earth elements is 5 parts by weight or less with respect to 100 parts by weight of alumina serving as the predominant component.”
Meanwhile, recent internal combustion engines to which a spark plug is attached are designed for a high-output mode or the like, and increase in area of an intake or an exhaust valve in a combustion chamber, employment of 4-valve configuration, etc. have been proposed. In this trend, downsizing (small-scale or small-diameter and thinning) of a spark plug itself and the insulator thereof is planned more and more. In addition, when such internal combustion engines have been designed for a high-output mode or the like, a spark plug, in particular an area surrounding the electrodes, is possibly exposed to high-temperature conditions which have never been experienced.
Therefore, it is important for such thinned insulators to have a satisfactory withstand voltage characteristic and mechanical strength not only in a high temperature (e.g., about 700° C.) environment but also in a higher temperature environment. However, no studies have been conducted for the spark plug insulator and the materials thereof disclosed in Patent All of references cited above have a satisfactory withstand voltage characteristic and mechanical strength in such a high temperature environment.